Safety issues regarding fuel cell vehicles and hydrogen fueled vehicles
I.
Introduction
The current discussion on dependence on foreign oil imports and the President’s
“Freedom Car” initiative has brought alternatively fueled vehicles and hydrogen to the attention
of the public. Even though all current fuel cell and hydrogen vehicles are experimental and only
being tested in low numbers on public roadways, several states have mandated zero emission
vehicles to be introduced (ZEV) in the near future.
As new technologies, fuel cells and hydrogen have some associated safety concerns that have to
be addressed. Although some of the concerns raised are similar to those encountered with
substances such as compressed or liquid natural gas (CNG or LNG) and technologies such as
electrically powered vehicles, the combination of different technologies in new vehicles and the
addition of new and unproven technology warrant special attention. First responders should be
informed of the potential risks of newly developed vehicles before they are commercialized, and
measures should be put in place to minimize the hazards to the general population.
II.
Properties of Hydrogen
Hydrogen has several properties that differ strongly from natural gas or methane.
Hydrogen is being used experimentally as a vehicle fuel, not only because it oxidizes to harmless
water, but also because it has a higher energy density per unit of weight than CNG or methane.
One of the other positive characteristics of hydrogen is that it disperses very quickly, meaning
that hydrogen concentrations under normal pressure dissolve to incombustible levels very
quickly. This also means that under ambient air pressure hydrogen has very little energy density
per unit of volume compared to other vehicle fuels. Hydrogen also rises very quickly and
therefore is less of a threat outdoors.
While hydrogen has a high burn/explosive velocity, it has less explosive power than other fuelair mixes. Except in extremely high concentrations, hydrogen is not toxic to humans. Hydrogen
is odorless and tasteless. While the flame of burning hydrogen is visible under daylight
conditions, it is a lot less so than flames from other fuels due to the lack of soot. A hydrogen
flame can be most easily identified by the mirage-like effect on the air over and around the
flame, as it otherwise does not produce significant heat radiation.
Hydrogen also has many characteristics that warrant its being handled with great care.
Hydrogen-air mixtures can ignite or explode at both lower and higher concentrations of the gas
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in the air than CNG or methane. Hydrogen is more easily ignited than other fuels. The impact of
this is negligible, however, as most other fuels can already be ignited by small amounts of static
electricity.
To store hydrogen in liquid form, it has to be cooled down dramatically. Hydrogen has a boiling
temperature of only 20° Kelvin or -435° F. This causes the fuel to boil off very quickly when
spilled, creating only a narrow window for ignition. On the other hand, the intensely cold fuel
can cause serious cold burn damage to people and embrittle and break metal equipment in
particular. In enclosed locations with normal temperatures, spilled liquid hydrogen will create
enormous gas pressures, tearing apart vessels without safety valves.
III.
Areas of Concern
The two prime dangers from fuel cell and hydrogen-powered vehicles are the danger of
electrical shock and the flammability of the fuel.
Fuel cells power vehicles by electro-chemically combining hydrogen gas (H2) and
oxygen (O2) from the surrounding air into water (H20) and electrical energy. The electrical
energy is then used to power both the locomotion of the vehicle through electrical motors and the
current electrical usage devices such as the radio, lights and air-conditioning. A notable
difference between current and new-technology vehicles is that the voltage needed to power the
electric motors is much higher in new vehicles than can be accommodated by the current
standard voltage of a 14V system; the automobile industry is in the process of moving to a new
standard of a 42V system. The 42V system was chosen as an industry standard in part for safety
reasons: anything greater than 50 volts can stop a human heart. On the other hand, some fuel
cell vehicle motors run on voltages exceeding 350V. With such high currents, the danger of
electric shock is great.
The second area of concern lies in the fuels used to power this future generation of
vehicles. Even though hydrogen remains the main focus of future fuel cell vehicles, it is neither
the only possible fuel for them (other fuels used to power fuel cells directly include methanol,
ethanol and methane), nor is hydrogen used only for this purpose. In addition, the hydrogen used
to power a vehicle does not necessarily have to be stored on the vehicle as hydrogen. Reforming
different hydrogen sources, such as alcohols, methane, propane and even regular gasoline, can
create gaseous hydrogen in the vehicle itself. Hydrogen stored as such in a vehicle or reformed in
it can also be used to power a ‘classic’ internal combustion engine. Besides reforming hydrogen
in the vehicle itself, there are several ways of storing hydrogen in a vehicle. Each has its own set
of flammability issues.
Both the electrical current and the flammability concern of the fuel translate into the
design needs for the vehicle itself as well as the requirements for structures intended for the
storage, refueling and repair of these vehicles.
A. The Vehicle Itself
Fuel cell vehicles currently being tested include public transportation and personal
mobility vehicles. According to the US Department of Energy (DOE), there were nine different
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hydrogen vehicle-testing projects underway in the US as of March 2003. The safety issues
regarding the vehicle can be divided into two separate categories. One category encompasses
issues with normal vehicle operations; the other category contains issues with vehicle accidents.
As stated above, the main issues with fuel cells and hydrogen-powered vehicles stem from
electrical shock and the flammability of the fuel.
1.
Electric current
The fact that over 350V are needed for the drive train of fuel cell vehicles presents both
an electrocution hazard and an ignition source for fuel contained in the vehicle or outside
materials. Since a significant amount of the material used in vehicular construction is metal, with
some degree of electrical conductivity, there is a high potential for electrical faults. This can pose
a threat both in normal operations of the vehicle and especially in accidents. Even though most
designs contain failsafe switches for the electrical system, these switches may be short-circuited
if the vehicle is involved in an accident.
In addition to the electric current generated by the fuel cell during its operation, most
prototype vehicles have an electrical storage component for acceleration and start up, much like
today’s hybrid vehicles. Most fuel cell vehicle store and draw on this additional electricity in
form of batteries. Batteries can also represent the additional danger brought on by the presence of
acids, to both the electrical system and the fuel system. More exotic and less researched forms of
energy storage are ultra capacitors and mechanical flywheels. Ultra capacitors store electrical
energy under high voltages for rapid release. While this is positive for vehicle operation, it also
holds the risk of very strong unintentional electrical discharges. Flywheels store energy as
movement energy of a rapidly revolving weighted object in an electromagnetic enclosure that
acts as an electrical motor and generator. If flywheels are unbalanced or their enclosures broken
by an accident, they can release massive amounts of physical energy on their surroundings.
2.
Electrical Drive System
The electrification of the drive system can also cause the vehicle to be a source of new
dangers. Electrical or computer faults can cause the vehicle to engage the motor by itself, reverse
the direction or engage the brakes. It remains to be seen if these dangers are greater than those
posed by current mechanical drive systems, but new technology generally needs some time to
find and address some technical quirks.
B.
Fuel-Specific Problems
Fuel cell vehicles can be fueled in a variety of different ways. Regardless of the source of the
fuel, the hydrogen, methane, methanol, or ethanol has to be stored and transported to the fuel cell
or engine.
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1.
Internal fuel transmission and consumption
Methanol and ethanol are both liquids and thus even though they are more flammable
than regular gasoline or diesel oil, they are more manageable than gaseous substances. Current
experience with conventional engines fueled with these two alcohols should be applicable to fuel
cells using them. Methanol has the added problem of being toxic.
Methane is the main component of natural gas (70%). The difference in the use of methane by
the fuel cell, as opposed to hydrogen, is that methane can produce carbon monoxide, which
besides being able to ‘poison’ the fuel cell can also poison occupants of the vehicle. Methane
fuel cells are in early stages of development with no current model uses in vehicles. Experience
with natural gas should be otherwise very much applicable to methane.
Thus, the main fuel-related issues for fuel cell safety regard the use of hydrogen. While being a
very clean and energy-dense fuel, hydrogen has the tendency to disperse quickly under normal
pressure. This causes the need for higher pressure of hydrogen in the fuel transport system than
for natural gas. Additionally, hydrogen molecules are so small that they can easily escape
through miniature holes and can even enter the molecular structure of some steels, making them
brittle over time. Also, the use of very fine membranes in Proton Exchange Membrane (PEM)
fuel cells can lead to direct combustion of hydrogen with oxygen. In normal operation of the
vehicle, slowly escaping hydrogen that collects to form a flammable or even explosive mixture
with air is the main matter of concern. An accumulation of gaseous hydrogen is seen as
particularly dangerous in the enclosed passenger or storage compartments of any hydrogenfueled vehicle.
2.
Fuel Storage
Storage of methanol and ethanol will be similar to that of today’s liquid fuels. Methane
storage can be virtually copied from natural gas storage either in compressed or liquid form.
Hydrogen storage again poses the main problem for fuel cell and hydrogen-using vehicles.
At normal pressure, hydrogen takes up a huge volume per unit of energy. This can be
addressed by creating hydrogen in the vehicle itself by reforming different hydrogen sources,
such as alcohols, methane, propane and even regular gasoline. Even though all these sources of
hydrogen have their own issues concerning fuel storage, there are established methods for storing
them. The reforming process, however, has its own risks, as it combines pressure and
temperature differences in an environment of moving volatile substances. There are numerous
approaches to reforming the different hydrogen-carrying substances. Most of these reforming
processes are still too energy intensive, hot or voluminous to be included in vehicles, particularly
in passenger cars. As reforming works around the most of the problems of hydrogen storage, it
has enormous potential.
Hydrogen can be stored in four main ways: compressed, as a liquid, in hydride form
bound to metals, or on the surface of solid porous materials or carbon nano-tubes. The forms of
storage that are currently being tested are compressed hydrogen and liquid hydrogen.
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Compressed hydrogen is stored at 3,600 psig, 5,000 psig or 10,000 psig. The first two of
these pressures are currently being tested. In contrast with compressed natural gas (CNG),
compressed hydrogen cannot be stored in steel tanks, as the hydrogen molecules would embrittle
the metal. Also the pressure in these tanks is much higher than in tanks for CNG. Even though
these tanks were and are being designed with extreme care, the possibility of a leak still exists.
This again might lead to the accumulation of flammable or even explosive hydrogen air
mixtures. The rupture of the pressure tank can cause very high concentrations of hydrogen to
form in the vicinity of the vehicle, as the turbulent flow rate of hydrogen is extremely high. Even
though hydrogen disperses very quickly, this emission will cause a combustible mix to form for a
short period in the open. Enclosed areas could accumulate enough hydrogen-air mixture for a
large explosion.
Liquid hydrogen (LH2) has to be stored at temperatures lower than 20° Kelvin or -435° F.
Even well-insulated storage tanks cannot maintain this low a temperature without relying on
outside cooling, which is prohibitive for passenger vehicles from an energy-needs standpoint.
The result of this is the expected leakage of 1%-3% of the hydrogen contained in the LH2 tank
per day, depending on the use and build of the vehicle. This controlled emission of hydrogen
over time is not considered overly dangerous, as controlled oxidization by catalysts or dispersion
is possible. Catastrophic ruptures of the LH2 tank can not only endanger people due to the
extreme temperature of the liquid, but can also lead to very high hydrogen concentrations in the
surrounding air, especially in confined places such as tunnels.
One of the safest methods of storing hydrogen in vehicles is by binding it with metal
hydrides. For this method of storage hydrogen, is bound to different metal alloys in porous and
sometimes loose form by applying moderate pressure and heat. The application of heat and the
reduction of the pressure are later used to extract the hydrogen gas from the metal. The greatest
drawback of this method is the weight of the metal hydride needed to contain sufficient fuel for
sustained vehicle operations (>200 mi). The weight of some alloys can go up to 1250 kg to store
15 kg of hydrogen, thereby greatly increasing the weight and also decreasing the vehicle’s
energy efficiency. Another potential problem with metal hydrides is the flammability of some of
the alloys used, such as magnesium in MgNi alloys, which have much better metal-to-hydrogen
rates than the example above.
Solid porous materials and carbon nano-tubes are still in the early stages of development.
While their properties appear to be close to metal hydride storage, they have other problems
related to the porous material, the high volume and weight of the material per weight of
hydrogen carried which are still unsolved. The carbon nano-tubes are now still prohibitively
expensive and also have the potential problem of flammability.
C. The Fueling and Maintenance of the Vehicle
Aside from being operated normally on public roads, fuel cell and hydrogen vehicles also
need to be parked, fueled and maintained. As most of the current hydrogen studies are being
conducted on a very low number of vehicles, their infrastructure is mostly located in a very
controlled environment. When these tests expand to ‘normal’ consumer trials, these vehicles
need a more public infrastructure available to them. This raises some associated safety concerns
with both new infrastructure for fueling and maintenance as well as private and public parking.
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In a U.S. electric code from the 1930s, hydrogen was given a very high flammability class that
corresponds to high requirements for electric appliances in the vicinity of hydrogen. A review of
this rather extreme classification of hydrogen is under way.
1.
Fueling
Both fuel cell vehicles requiring other types of fuel and hydrogen-powered vehicles will
need a specialized infrastructure to maintain their operations. As for the issues for the vehicle
fuel system, this paper focuses on hydrogen, since the other potential fuels are already being
distributed for fleets of vehicles or are similar to current fuels. Hydrogen today is transported as
a liquid in insulated trailers similar to fuel trucks. Future sources hydrogen for fueling stations
can be gaseous hydrogen pipelines or reforming hydrogen on-site from the source-fuels
described above. Aside from an injection fueling mechanism for the vehicles, there are issues
with the storage of hydrogen at the fueling sites and the detection of hydrogen leaks. Especially
with fuel cell vehicles, there is a great potential for problem with the electric circuitry in the
vicinity of flammable gases.
2.
Maintenance
The main problem for maintenance localities for fuel cell and hydrogen-fueled vehicles is
the electric code mentioned above. Especially in connection with the high voltage and complete
electrification of these vehicles, this will definitely pose a problem. Aside from this, there are the
general problems of stationing a vehicle in an enclosed space because of the possibility of a
buildup of gaseous hydrogen.
a)
Parking of the vehicle
Parking a hydrogen vehicle or other gas-fueled vehicle in an enclosed structure is a
serious safety concern as it can lead to a buildup of the gas. Hydrogen’s tendency to rise and
disperse rapidly makes this the only situation in which small leaks can create extremely
dangerous situations.
b)
Emission
As noted above, liquid hydrogen tanks always emit small quantities of hydrogen if it is
not oxidized or burned off in a controlled manner. All hydrogen-containing fuel systems have a
great propensity for at least small leaks due to the pressure of the gaseous hydrogen and the
small size of the individual molecules. Because of the high dispersion rate of hydrogen, small
leaks would likely pose a problem only in small individual garages. Safety concerns in larger
private or public parking garages will be more of an issue if and when the number of hydrogenfueled vehicles rises dramatically. Without major changes to parking structures or installation of
continuous ventilation even in private, individual garages, hydrogen detectors will be essential to
protecting all enclosed environments in which hydrogen will be present.
3.
Detection
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The high probability of at least trickling emissions of hydrogen and the lower flammability level
of about 6% (hydrogen under normal pressure) leads to the necessity of early detection of even
very low concentrations of hydrogen. Sensors to detect concentration of hydrogen below the
lower flammability level are currently still very expensive. Odorants that are added to natural gas
cannot be easily added to hydrogen or methane used in fuel cells as the larger molecules and
especially the sulfur content of current odorants would poison fuel cells. Research is being
conducted into the possibility of removing these odorants before the fuel enters the fuel cell. This
would leave the fuel cell itself and molecular-sized leaks in the fuel transport system as the only
sources of odor-free hydrogen. Odorants are still difficult for detectors to pick up, but they are
less expensive than hydrogen detectors.
IV.
Current Status of Regulation / Model Codes
In the US, the Bush Administration’s fuel cell and hydrogen initiative and increasing testing of
fuel cell vehicles has spurred diverse regulatory activity. The DOE Energy Efficiency and
Renewable Energy Office’s Hydrogen, Fuel Cells & Infrastructure Technologies Program is
trying to coordinate much of this development.
•
The National Fire Protection Association (NFPA) is currently reviewing its standards for
hydrogen vehicle fueling and maintenance stations (NFPA 50A and NFPA 50B).
•
The Society of Automotive Engineers (SAE) is in the process of drafting guidelines for
hydrogen fueled and fuel cell vehicles.
•
Also involved are the International Code Council (ICC), which is reviewing its building
codes; the American Society of Mechanical Engineers (ASME) and the American Society for
Nondestructive Testing (ASNT) on the safety and testing of the fuel tanks; and several other
hydrogen, gas and testing institutions.
Internationally, the World Forum for the Harmonization of Vehicle Regulations, WP.29 of the
United Nations’ Economic Commission for Europe, has formed an informal group to develop
safety regulations for hydrogen-fueled vehicles. In March 2003, at a meeting of the Forum,
Jeffrey W. Runge, M.D., administrator of the National Highway Transportation Safety
Administration of the US Department of Transportation, addressed the safety of hydrogen-fueled
vehicles in the following way:
“[I]n our exuberance about the advantages for our environment and consumption of fossil
fuels, we must not be caught off guard with the potential impact on safety. It will be
challenging to find the appropriate balance between ensuring a continuing energy supply,
protecting the environment, and ensuring safety. Only then can we move forward with
confidence, and in turn, build consumer confidence in these advanced technologies.
Many countries around the world have been conducting research to promote hydrogen
vehicle technologies. Just last month, President Bush, in his annual ‘State of the Union’
address to America, announced his intentions to focus U.S. efforts on developing, testing
and deploying hydrogen-fueled vehicles. The industry worldwide has made tremendous
progress in researching and building prototypes of these vehicles. I have personally
driven hydrogen fuel cell vehicles in Europe, Japan and the U.S., and I find them to be
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promising technology. However, thus far, our safety evaluation of these vehicles is
limited. While the industry has been working diligently to develop harmonized industry
standards, governments are only beginning to assess these technologies from a regulatory
perspective. Much research and testing is still needed in order to evaluate their safety
impact and develop a performance-oriented regulation that will not limit technological
innovation.”
V.
Non-vehicular developments
Outside the use of fuel cells in vehicles, there is an increasing trend especially in the
mobile computing and consumer electronics field to substitute fuel cells for batteries. This is
especially true for power-hungry devices such as notebook computers and video cameras. Some
prototypes for laptops already exist, and regulations for carrying hydrogen on airplanes are being
discussed. Even though the amounts of hydrogen used in these applications is very small, this
development seems to warrant some attention.
VI.
Sources
American Methanol Institute, ‘Beyond the Internal Combustion Engine’
http://www.methanol.org/fuelcell/special/promise.cfm
College of the Desert ‘Hydrogen Fuel Cell Engines and Related Technologies’ Dec. 2001
http://www.ott.doe.gov/otu/field_ops/pdfs/fcm02r0.pdf
DOE ‘Hydrogen and Fuel Cell Resource Guide’ Module 2
www.afdc.doe.gov/pdfs/hy-fc_rg.pdf
DOE Energy Efficiency and Renewable Energy Office web site
http://www.eere.energy.gov/
DOE Energy Efficiency and Renewable Energy Office, ‘Fuel Cells for Transportation, Program
implementation Strategy’ Dec. 1997
http://www.ott.doe.gov/pdfs/fueljune.pdf
DOE Federal Technology Alert – Natural Gas Fuel Cells
http://www.pnl.gov/fta/5_nat.htm
DOT ‘Clean Air Program: Design Guidelines for Bus Transit System using Hydrogen as an
Alternative Fuel, Office of Research, Demonstration, and Innovation,’ PDF April 1999
http://transit-safety.volpe.dot.gov/Publications/CleanAir/BTS/BTSDesignGuidelines.pdf
DOT ‘Revision to Regulations Governing Transportation and Unloading of Liquefied
Compressed Gases’ CFR Vol. 64, No. 54 March 22, 1999.
http://www.fmcsa.dot.gov/rulesregs/fmcsr/final/032299.pdf
Ford Motor Company Report for DOE on ‘Direct-Hydrogen-Fueled Proton-ExchangeMembrane Fuel Cell Systems for Transportation Applications’ May 1997 DOE/CE/50389-502
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Idaho National Engineering and Environmental Laboratory,
‘Safety Issues with Hydrogen as a Vehicle Fuel’ prepared for DOE Sept. 1999
http://www.inel.gov/x-web/other/framed.shtml?/energy/fossil/hydrogen/pdf/h2safetyreport.pdf
Mechanical Engineering Magazine ‘Fluid Power and Processing’ Feb 02.
http://www.memagazine.org/backissues/feb02/features/fillerup/fillerup.html
Methanex Corporation, ‘Human Exposure to Methanol’
http://www.methanex.com/fuelcells/healthandsafety/humanexposure.pdf
Making Hydrogen by Electrolysis of Methanol, NASA Tech Brief Vol. 26, No.6
http://www.methanol.org/pdfFrame.cfm?pdf=JPL_DMFC.pdf
National Hydrogen Association Web Site
http://www.hydrogenus.com/
NFPA web site
http://www.nfpa.org/ecommittee/hcgroup/hcgroup.asp
Renewable Fuels Association, ‘Ethanol and Fuel Cells: Converging Paths of Opportunity‘
http://www.ethanolrfa.org/RFA_Fuel_Cell_White_Paper.PDF
Runge, Jeffrey W., M.D., administrator, National Highway Traffic Safety Administration, US
Department of Transportation, remarks at the World Forum for the Harmonization of Vehicle
Regulations, United Nations’ Economic Commission for Europe, Geneva, Switzerland, 12
March 2003.
SAE draft J2578: Recommended Practice for General Fuel Cell Vehicle Safety
VII.
General Hydrogen/Fuel Cell/Safety Websites
http://www.sae.org/fuelcells/
http://www.hydrogensafety.info/
http://www.fuelcells.org/
http://www.h2cars.de/
http://www.eere.energy.gov/hydrogenandfuelcells/hydrogen/related_links.html
http://www.eere.energy.gov/hydrogenandfuelcells/fuelcells/related_links.html
http://www.eere.energy.gov/hydrogenandfuelcells/codes/related_links.html
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